Nanocomposites based on a modified starch and EVOH were prepared in the present study. The starch was modified with ionic groups (acetate sodium group or trimethylammonium chloride group) and hydrophobic groups (hexanoyl group or benzoyl group). The rationale behind the approach was to introduce ionic interaction between the modified starch and nanoclay, so that the nanocomposites could have a high degree of dispersion of nanoclay aggregates.
The dispersion characteristics of nanocomposites based on the anionically modified starch (AS-B or AS-LF) or the cationically modified starch (CS-B or CS-LF) were investigated by XRD and TEM. For the nanocomposites based on AS-B or AS-LF, the experimental results have shown that organoclay aggregates (Cloisite 30B and Cloisite 15A) have a high degree of dispersion characteristics, whereas natural clay (MMT) aggregates have very poor dispersion characteristics. We have ascribed the high degree of dispersion of organoclay aggregates in nanocomposites based on the anionically modified starch and EVOH to the ionic interaction formed between the negatively charged carboxylate groups (−COO−) in the modified starch and positively charged N+ in a surfactant (MT2EtOH or 2M2HT) residing at the surface of layered silicates of organoclays. The FTIR spectra of nanocomposites confirmed the presence of ionic interaction in the nanocomposites. On the other hand, there is no functional group residing at the surface of MMT to form any specific attractive interaction with the matrix in the nanocomposites, which we ascribed to the poor dispersion characteristics of MMT aggregates.
For the nanocomposites based on the cationically modified starch (CS-B or CS-LF) and EVOH, the experimental results have shown that both organoclays (Cloisite 30B or Cloisite 15A) and natural clay (MMT) gave rise to poor dispersion characteristics in nanocomposites. We have interpreted the results by presenting evidence that no attractive interaction existed between the clay and the cationically modified starch. However, we have learned that the nanocomposite based on the same cationically modified starch, which was prepared by solution blending, has a very high degree of dispersion of MMT aggregates. We ascribed the high degree of dispersion of MMT aggregates in the nanocomposite to the ionic interaction between the negatively charged surface of MMT and positively charged −N+ (CH3)3 in the modified starch. The FTIR spectra confirmed the presence of ionic interaction in the nanocomposite. Then we explained why ionic interactions existed between the MMT and cationically modified starch in the nanocomposites prepared by solution blending.
The nanocomposites prepared in the present study were characterized by DSC and WAXD. The glass transition temperature was increased in those nanocomposites which have a high degree of dispersion of organoclay aggregates. We ascribed the increased glass transition temperature to the ionic interaction between the aggregates of organoclay and matrix in the nanocomposites. The DSC thermograms showed that the melting point and melting enthalpy of EVOH in the nanocomposites containing the aggregates of Cloisite 30B were increased, suggesting that the dispersion characteristics of the organoclay influenced the crystallization of EVOH in the nanocomposites. The WAXD patterns have shown that in the nanocomposites containing exfoliated Cloisite 30B, the crystalline structure of EVOH takes orthorhombic lattice in the quenched and annealed samples at various annealing temperatures. In the nanocomposites based on EVOH and a modified starch, and Cloisite 15A or MMT, the crystalline structure of EVOH was found to be similar to that of neat EVOH. We ascribed the difference in crystallization and the crystalline structure of EVOH between the nanocomposite containing Cloisite 30B and the nanocomposites containing other clays (Cloisite 15A and MMT) to the presence of hydrogen bonds formed between the hydroxyl group in EVOH and the hydroxyl group in the surfactant (MT2EtOH) residing at the surface of silicate sheets of Cloisite 30B.
The tensile properties of the nanocomposites prepared in the present study were characterized. We have found that an improvement in the tensile properties of the nanocomposites based on a modified starch and EVOH can only be obtained if the clay has a very high degree of dispersion in the nanocomposites, and strong attractive interaction is present between the silicate sheets of clay and the matrix.